Dew resistant coatings

ABSTRACT

The present invention relates to dew resistant coatings and articles having the dew resistant coating adhered thereto. The dew resistant coatings comprise elongate silica particles. These coating are useful on articles or surfaces used in outdoor applications and articles and surfaces used in moist indoor environments.

This application claims the benefit of provisional application Ser. No.60/562,488 filed on Apr. 15, 2004, which is hereby incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to dew resistant coatings and articleshaving the dew resistant coating adhered thereto. The dew resistantcoatings are useful on articles or surfaces used in outdoor applicationsand are particularly useful on retroreflective articles.

BACKGROUND OF THE INVENTION

There exists a need for imparting dew resistance to transparentsubstrates such as windshields, lenses, goggles, and windows, andreflective substrates such as mirrors and retroreflective traffic signs.While retroreflective traffic signs currently provide optimum levels ofheadlight reflectivity to motorists, accumulation of dewdrops on thesurface of the retroreflective sign can result in potentiallycatastrophic “blackouts” in which the signs are ineffective in providingvital information to motorists. This problem has been described by JohnA. Wagner in the Florida Sate Department of Transportation Report “HPRResearch Study M29-82”, October 1989. In certain parts of the world, theclimate is such that moisture from the atmosphere readily condenses ontosurfaces when the temperature of the surface drops below the dew point,the temperature at which the air is fully saturated with water vapor andbelow which precipitation of water in the form of “dew” occurs. Whenformed on the surface of mirrors and retroreflective surfaces, thesedewdrops scatter the incident light, resulting in the loss ofreflectivity or “blackouts”.

One method of preventing condensation and the formation of dewdrops isto heat the surface of the substrate to a temperature above the dewpoint. U.S. Pat. No. 5,087,508 describes the use of phase changematerials in a thermal reservoir located behind the outer layer of adisplay sign. The phase change material undergoes at least one phasechange, e.g., from liquid to solid state or from one crystalline stateto another, between about −20° C. and about 40° C. During periods offalling ambient temperature, the thermal reservoir will yield heat,thereby warming the outer layer of the display sign. European PatentApplication 155,572 describes a device for preventing the formation ofdew and frost on retroreflective road sign carriers in which a thermalradiator is arranged above and in front of the road sign. Neither ofthese devices provides a complete solution to the problems associatedwith the formation of dew. The device of U.S. Pat. No. 5,087,508requires “recharging” of the phase change material at highertemperatures, while the device of EP 155,572 simply minimizes dewformation by minimizing radiative cooling of sign surfaces to the nightsky.

Surfactants have been used to obtain anti-fog properties on the surfaceof polymer films. The surfactants used are generally small molecules oroligomeric in nature, and present in relatively low concentrations.Examples of surfactants used for anti-fog applications in food packagingand greenhouse products include those described in U.S. Pat. Nos.4,136,072; 4,609,688; 5,451,460; 5,766,772; 5,846,650; and 6,296,694 andEP 1,110,993. In general, the surfactant coatings are susceptible towater washing due to the low concentrations of surface active molecules.In addition, many of the anti-fog films are not dew resistant andexhibit only a modest decrease in surface water contact angles.

Polymeric forms of hydrophilic surface agents have been disclosed asbeing useful for anti-fog films. U.S. Pat. No. 5,877,254 describes ananti-fog and scratch resistant polyurethane composition that include anisocyanate prepolymer, a hydrophilic polyol and an isocyanate-reactivesurfactant. U.S. Pat. No. 4,080,476 describes an anti-fog coating foroptical substrates wherein the coating comprises a polymerized monomerof, for example, 2-acrylamido-2-methyl propane sulfonic acid.International Publication WO 99/07789 describes the use of siloxanederivatives of polyetheralcohols as an anti-fog additive to a polyolefinprior to formation of a polyolefin film. Many of the prior art coatingsdo not provide a consistent long-lasting anti-fog coating. Rather, theanti-fog properties of these coatings fail after repeated washings withwater.

SUMMARY OF THE INVENTION

A dew-resistant coating having particular utility for retroreflectivearticles is described. The dew-resistant coating is obtainable from afilm-forming inorganic or inorganic/organic hybrid compositioncomprising silica wherein the silica particles comprise elongateparticles having an aspect ratio of greater than 1. In one embodiment,the aspect ratio is greater than 2.

In one embodiment the invention is directed to a dew-resistant coatingcomprising at least about 75% by weight of elongate silica particleshaving a width of about 9 to about 15 nanometers and a length of about40 to about 300 nanometers. The coating may optionally include anorganic binder.

The dew resistant coatings of the present invention are useful forapplications that include, but are not limited to, retroreflective andgraphic signage, automotive interior glass, transportation industrypaint, i.e., aviation, train and automobile paint, boat and shipbottoms, lubricous pipe coatings, freezer windows, clear plasticpackaging, chromatography support, medical equipment surface treatment,bathroom mirrors, shower enclosures, and eyeglasses.

One embodiment of the invention is directed to a retroreflective articlecomprising a substrate and a coating provided on at least a portion of asurface of the substrate that is exposed to moist air, the coatedportion being retroreflective and the coating comprising elongate silicaparticles having an aspect ratio of greater than 1. Retroreflectivearticles to which the coating of the present invention may be appliedinclude raised pavement markers having one or more retroreflectiveelements on the surface, traffic signs, license plates or self-adhesivestickers bearing visually observable information. In one embodiment, thecoating on at least a portion of the retroreflective article comprisesat least 75% by weight of elongate silica.

Another embodiment of the invention is directed to a method of impartingdew-resistance to a retroreflective article, the method comprising:providing a retroreflective article having a surface; preparing acoating composition comprising elongate silica particles having a widthof about 9 to about 15 nanometers and a length of about 40 to about 300nanometers; applying the coating composition to at least a portion ofthe surface of the retroreflective article; and heating the coatingcomposition to form a dew-resistant coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an apparatus used to measure dewresistance of an article having the dew resistant coating of theinvention coated thereon.

FIG. 2 is a gel permeation chromatography trace of various hydrolyzateoligomers of 3-glycidoxy propyltrimethoxy silane useful in the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The dew-resistant coating of the present invention comprises elongatesilica fine particles. The elongate silica particles are coated orgrafted onto the surface of the substrate. The substrate is generallyglass or a polymeric film. The dew-resistant coating can be transparent.When the substrate to which the dew-resistant coating is applied is aretroreflective film, a transparent dew-resistant coating is required.

The silica useful in the present invention comprises elongate particlesilica having an aspect ratio that is greater than 1.0. In oneembodiment, the aspect ratio is greater than 2.0. As used herein, theterm “aspect ratio” means the ratio of the length of the particle to thewidth. In one embodiment, the silica particles have an average width(diameter) of about 9 to about 15 nanometers and an average length ofabout 40 to about 300 nanometers. The elongate silica particles may bedispersed in an alcohol or in water. Commercially available elongatesilica includes those available from Nissan Chemical Industries underthe trade designations SNOWTEX UP and SNOWTEX OUP. The silica may alsocomprise string-of-pearls silica particles, which are chain silicaparticles available from Nisson Chemical under the trade designationSNOWTEX-PS. The solvent in which the particles are dispersed may bewater, methanol, ethanol, isopropropanol, etc.

In one embodiment, the coating composition comprises at least 75% byweight fine elongate silica particles. In other embodiments, the coatingcomposition comprises at least 80%, or at least 90%, or at least 95% byweight fine elongate silica particles. In one embodiment, the coatingcomposition comprises fine elongate silica particles and fine sphericalparticles having an average diameter of less than about 50 nanometers.The spherical particles may be provided in a colloidal dispersion ofsilica in a solvent that is compatible with the solvent of the elongatesilica particles. For example, the spherical silica particles used maycomprise Snowtex IPA-ST-MA from Nissan Chemical Industries, which is asilica sol of spherical silica particle having an average particlediameter of about 17-23 nanometers dispersed in isopropyl alcohol. Auseful ratio of elongate silica particles to spherical silica particlesis in the range of about 100:0 (i.e. 100% elongate silica) to about70:30. In one embodiment, the ratio of elongate silica particles tospherical particles is in the range of about 100:0 to about 90:10.

In one embodiment of the present invention, the coating compositioncomprises fine elongate silica particles and an organic binder. Theorganic binder may be present in an amount of about 0 by weight to about10% by weight based on the total solids of the coating composition. Inone embodiment, the organic binder may be present in an amount of about4% to about 8%, or about 15% to about 25% by weight. The organic bindermay comprise hydrolysis products and partial condensates of one or moresilane compounds. Useful silane compounds include, but are not limitedto epoxy-functional silanes. Examples of such epoxy-functional silanesare glycidoxy methyltrimethoxysilane, 3-glycidoxypropyltrihydroxysilane,3-glycidoxypropyl-dimethylhydroxysilane,3-glycidoxypropyltrimeth-oxysilane, 3-glycidoxypropyl triethoxysilane,3-glycidoxypropyl-dimethoxymethylsilane,3-glycidoxypropyldimethylmethoxysilane,3-glycidoxypropyltributoxysilane, 1,3-bis(glycidoxypropyl)tetramethyldisiloxane, 1,3-bis(glycidoxypropyl)tetramethoxydisiloxane,1,3-bis(glycidoxypropyl)-1,3-dimethyl-1,3-dimethoxydisiloxane,2,3-epoxypropyl-trimethoxysilane, 3,4-epoxybutyltrimethoxysilane,6,7-epoxyheptyl-trimethoxysilane, 9,10-epoxydecyltrimethoxysilane,1,3-bis(2,3-epoxypropyl) tetramethoxydisiloxane,1,3-bis(6,7-epoxyheptyl)tetramethoxydisiloxane,2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, and the like.

Other useful silanes include methyltrimethoxysilane,ethyltrimethoxysilane, propyltrimethoxysilane, butyltrimethoxysilane,isobutyltrimethoxysilane, hexyltrimethoxy silane, octyltrimethoxysilane,decyltrimethoxysilane, cyclohexyltrimethoxysilane,cyclohexylmethyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane,vinyltrimethoxysilane, allyltrimethoxysilane, dimethyldimethoxysilane,2-(3-cyclohexenyl)ethyltrimethoxysilane, 3-cyanopropyltrimethoxysilane,3-chloropropyltrimethoxysilane, 2-chloroethyltrimethoxysilane,phenethyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane,3-aminopropyltrimethoxysilane, phenyltrimethoxysilane,3-isocyanopropyltrimethoxysilane,N-(2-aminoethyl)-3-aminopropyltrimethoxysilane,4-(2-aminoethylaminomethyl)phenethyltrimethoxysilane,chloromethyltriethoxysilane, 2-chloroethyltriethoxysilane,3-chloropropyltriethoxysilane, phenyltriethoxysilane,ethyltriethoxysilane, propyltriethoxysilane, butyltriethoxysilane,isobutyltriethoxysilane, hexyltriethoxysilane, octyltriethoxysilane,decyltriethoxysilane, cyclohexyl-triethoxysilane,cyclohexylmethyltriethoxysilane, 3-methacryloxypropyltriethoxysilane,vinyltriethoxysilane, allyltriethoxysilane,[2-(3-cyclohexenyl)ethyltriethoxysilane, 3-cyanopropyltriethoxysilane,3-methacrylamidopropyltriethoxysilane, 3-methoxypropyltrimethoxysilane,3-ethoxypropyltrimethoxysilane, 3-propoxypropyltrimethoxysilane,3-methoxyethyltrimethoxysilane, 3-ethoxyethyltrimethoxysilane, 3-propoxyethyltrimethoxysilane,2-[methoxy(polyethyleneoxy)propyl]heptamethyl trisiloxane,[methoxy(polyethyleneoxy)propyl]trimethoxysilane,[methoxy(polyethylene-oxy)ethyl]trimethoxysilane,[methoxy(polyethyleneoxy) propyl]triethoxysilane,[methoxy(polyethyleneoxy)ethyl]triethoxysilane, and the like.

The organic binder may comprise a polymer that is hydrophilic.

The dew resistant coating of the present invention may comprise amonolayer or a multilayer coating. In one embodiment, a tie layer isapplied to the substrate to improve the adhesion of the outer silicacontaining coating.

In one embodiment, the dew resistant coating comprises a first layercomprising fine spherical particles and an organic binder and a second,outer layer, comprising fine elongate silica. In another embodiment, thedew resistant coating comprises a first layer comprising fine elongatesilica and an organic binder and a second, outer layer comprising aphotocatalytic layer.

The photocatalytic layer generally comprises TiO₂ particles.Photocatalytic compositions are disclosed in U.S. Pat. Nos. 6,228,480and 6,407,033 to Nippon Soda Company, the disclosures of which areincorporated by reference herein. The second photocatalytic layeraffords additional self-cleaning properties along with increasedhydrophilicity upon UV irradiation.

In one embodiment, photocatalytic nanoparticles are incorporated into adew resistant coating composition that is applied to the substrate in amonolayer. The coating composition may comprise metal oxide particles inaddition to the fine elongate silica particles. Such metal oxideparticle may be used to obtain a desired refractive index or to obtaindesired photoactivity. The elongate silica particles may be used incombination with other metals or metal oxides such as titania, zirconia,tin oxide, antimony oxide, iron oxide, lead oxide, needle TiO₂, bayerite(Al(OH)₃) and/or bismuth oxide to incorporate other adjunct propertiesincluding color, conductivity (thermal and/or electrical), abrasionresistance, etc.

In one embodiment, the coating composition comprises elongate silicaparticles, an organic binder and at least one surfactant. Usefulsurfactants include alkoxy siloxane-based surfactants, ethoxylated fattyalcohols such as lauryl alcohol, myristyl alcohol, palmityl alcohol andstearyl alcohol; polyethylene oxides; block copolymers of propyleneoxide and ethylene oxide; alkyl polyethoxy ethanols; polyethylene laurylether; polyethylene stearate; ethoxylated nonylphenol; sorbitan ester offatty acid; polyethylene sorbitan monostearate; polyglycerol esters offatty acids such as lauryl acid, palmetic acid, stearic acid, oleicacid, linoleic acid and linolenic acid; polyoxyethylene distearate;polyoxyethylene sorbitan tristearate; ethylene glycol monostearate;sodium lauryl ether sulfate; ethoxylated amine; ethoxylated acetylenicalcohol; sodium sulfosuccinate; sodium dodecyl benzene sulfonate;fluorosurfactants; acetylenics and combinations of two or more thereof.The surfactant may be present in an amount of 0 to 10% by weight of thecoating composition.

When an organic binder is used, the coating composition may be cured viafree radical, thermal, infrared, electron beam or ultraviolet radiationpolymerization. For UV curable compositions, useful photoinitiatorsinclude sulfonium or iodonium salts such as SARCAT CD1010, SARCAT CD1011and SARCAT CD1012 (available from Sartomer) and CYRACURE UVI 6974available from Dow Chemical, IRGACURE 651, 184 and 1700 and DAROCURE1173, available from CIBA-GEIGY; as well as GENOCURE LBP available fromRahn; and ESACURE KIP150 available from Sartomer;[4-[(2-hydroxytetradecyl)oxy]-phenyl]phenyliodoniumhexafluoroantimonate, benzophenone, benzyldimethyl ketal,isopropyl-thioxanthone, bis(2,6-dimethoxybenzoyl)(2,4,4-trimethylpentyl)phosphineoxide, 2-hydroxy-2-methyl-1-phenyl-1-propanone,diphenyl(2,4,6-trimethybenzoyl) phosphine oxides, 1-hydroxycyclohexylphenyl ketone,2-benzyl-2-(dimethyl-amino)-1-4-(4-morpholinyl)phenyl-1-butanone,alpha,alpha-dimethoxy-alpha-phenylacetophenone,2,2-diethoxyacetophenone, 2-methyl-1-4-(methylthio)phenyl-2-(4-morpholinyl)-1-propanone,2-hydroxy-1-4-(hydroxyethoxy)phenyl-2-methyl-1-propanone.Photosensitizers may be used in combination with the photoinitiator.Examples of photosensitizers include phthalimide derivatives,isopropylthioxanthone and carbazole compounds.

The coating composition of the present invention can be applied tosubstrates by conventional methods, including flow coating, spraycoating, curtain coating, dip coating, spin coating, roll coating, etc.to form a continuous surface film or as a pattern, as desired. In oneembodiment, the coat weight of the applied coating is about 1 gsm orless.

Any substrate compatible with the coating composition can be coated withthe dew resistant coating. For example, plastic materials, wood, paper,metal, glass, ceramic, mineral based materials, leather and textiles maybe coated with the dew resistant coating. Plastic substrates to whichthe dew resistant coating of the present invention may be appliedinclude acrylic polymers, poly(ethyleneterephthalate), polycarbonates,polyamides, polyimides, copolymers of acrylonitrile-styrene,styrene-acrylonitrile-butadiene copolymers, polyvinyl chloride,butyrates, polyethylene and the like. Transparent polymeric and glassmaterials coated with these compositions are useful as flat or curvedenclosures, such as windows, liquid crystal display screens, skylightsand windshields, especially for transportation equipment.

The dew resistant coating composition is particularly useful whenapplied to retroreflective sheeting. The transparent dew resistantcoating enables the underling retroreflective sheeting to maintain itsretroreflectivity and decreases or eliminates the likelihood of a“blackout” condition in moist environments. This may be achieved byusing a dew resistant coated glass or a transparent plastic film that isadhered as an overlaminate to a retroreflective sign. Alternatively,this may be directly incorporated onto the top surface of aretroreflective article. Retroreflective substrates include raisedpavement markers having one or more retroreflective elements on thesurface, traffic signs, license plates or self-adhesive stickers bearingvisually observable information.

In one embodiment, the dew resistant coating is applied to at least aportion of the surface of a retroreflective sheet. The surface of theretroreflective sheet may comprise an acrylate polymer. In oneembodiment, the surface of the retroreflective sheet to which the dewresistant coating is applied comprises a butylacrylate/methylmethacrylate copolymer.

In one embodiment, a removable protective layer is applied over the dewresistant coating to prevent damage to the dew resistant coating duringstorage, transport and application to the underlying substrate. Theremovable protective layer may comprise a polymeric film. In oneembodiment, the protective layer comprises a water soluble or watermiscible polymeric coating. Examples of such polymers includepolyethylene oxide, polyvinyl alcohol, polyacrylic acid, alkyl metalsilicate, polyvinyl pyrrolidone, (poly)hydroxyethyl methacrylate, andcombinations thereof.

TESTING METHODS

Coat Thickness

The coat thickness for film samples coated onto plastic substrates aredetermined via the cross-section method wherein a 2 micrometer thickslice is cut in the traverse direction through the dew-resistant coatingand the film support using a microtome (RMC Rotary Microtome MT 990)equipped with a diamond knife (Delaware Diamond Knife). The microtome isset to operate at −10° C. and a cutting speed of about 10 mm/sec. AnOlympus BX 60 optical microscope is used to observe the cross-sectionand to measure the coat thickness in micrometers via a digital camera(resolution 800×600) and the software package Image Pro-Plus at totalmagnification of 1000×. A second method used to determine coat weight isX-ray florescence spectrometry, which measures silicon for asilica-based coating, aluminum for an alumina-based coating, etc. Abench-top Oxford Lab-X 3000 XRF analyzer (Oxford Instruments) is used tomeasure dry coat weight of silica based coatings. Coated samples aredie-cut into 3.5 cm diameter disks to measure the quantity of siliconpresent.

Contact Angle:

The contact angle between the surface of the coated substrate and adroplet of water is an indicator of the hydrophilicity of the coating.The lower the contact angle, the better the hydrophilic properties ofthe coating. The hydrophilicity of the film surface is measured using anFTA 200 dynamic contact angle goniometer available from First TenAngstroms Corp. equipped with a Pelco video camera (PCHM 575-4). Contactangle measurements are taken using a 4 microliter water droplet inambient air humidity at time intervals of 1 second, 5 seconds, and 10seconds.

Water Wash Resistance

Coated samples are put in a bottle containing water and placed on amechanical roller (available from Norton Chemical Process ProductsDivision, Akron, Ohio) for 12 hours. Samples are then removed, re-washedunder running water, dried and tested appropriately for anti-fog, dewresistance and/or contact angle (hydrophilicity).

Anti-Fog:

The anti-fog property of the coatings is screened by blowing a breath ofair onto the surface of the test sample to determine if any hazedevelops. The sample may also be evaluated by putting the test surfaceface down, 1 inch away from the top of a boiling beaker of water. If nohaze or dew is observed after 30 seconds, the sample is rated to haveanti-fog properties.

Dew Resistance:

An outdoor dew resistance testing apparatus schematically represented inFIG. 1 is used to measure dew resistance outdoors. The dew-resistantcoated retroreflective samples are laminated onto a metallic trafficsign. Dew is generated naturally by heat loss to the atmosphere providedby satisfactory meteorological conditions. The digital camera (IQEye3camera server) is accompanied by an axial illumination system (12V whiteLED array) and a Rotronic 3 meteorological terminal that is linked tothe camera server via a local serial bus. The outdoor dew testertransmits, at specified time intervals, images and meteorological dataincluding dew point, relative humidity and air temperature to a dataserver via TCP communication protocol and the Internet. Reflectivitydata for the dew resistant coated retroreflective samples is integratedfrom the bitmap histograms and compared to a sample of the uncoatedretroreflective sample and plotted versus time. Each reflectivity dataset includes measured meteorological parameters such as air temperature,air relative humidity and dew point.

Percent Blackout

For a given dew event, the percent blackout of a dew resistant coatedsign is calculated as the number of hours the sign loses its reflectiveproperties (loses more than 50% of its original reflectivity) divided bythe number of hours the control (uncoated sign) loses its reflectiveproperties, multiplied by 100. The apparatus used to measure thereflective properties of the sign is described above with reference toOutdoor Dew Resistance testing.

Xenon Weathering:

Xenon weathering testing is carried out with an Atlas Ci5000 Xenon ArcWeather-Ometer (Atlas Electric Devices Company, Chicago, Ill.) accordingto ASTM G155-1 with two light cycle segments. For both light cycles,irradiance is the same: 0.35 watts/M² at 340 nm, with black paneltemperature set at 63° C., chamber temperature at 40° C., and relativehumidity at 50%. The first light cycle is 102 minutes, with no waterspray and the second light cycle is 18 minutes with water spray on thesample surface.

QUV Weathering:

A QUV Accelerated Weathering Tester (Q-Panel Lab Products, Cleveland,Ohio) is used to carry out the testing according to ASTM G-154procedures. A UVA-340 lamp with typical irradiance of 0.77 watts/m² or aUVB-313 lamp with typical irradiance of 0.63 watts/m² is applied in thetest. The UVA-340 lamp has similar spectral power distribution assunlight. The typical cycles include an 8 hour UV light cycle at 60° C.black panel temperature and a 4 hour condensation cycle at 50° C. blackpanel temperature.

Retroreflectivity:

A hand-held RetroSign retroreflectometer type 4500 (Danish Electronics,Light & Acoustics of Denmark) is used to measure retroreflectivityaccording to ASTM D4956-01 Standard Specification for RetroreflectiveSheeting for Traffic Control. The retroreflectometer measures with afixed entrance angle at −4° C. and observation angle of 0.2°C.

Mandrel Test:

The Mandrel test accelerates cracking of the coating, which contributesto increased haze, and therefore, decreases retroreflectivity.Retroreflective sheeting samples with adhesive backing are cut in 2 cm(cross direction)×4 cm (machine direction) strips and applied to a glassrod having a 1 inch diameter. Hand applied pressure is used to wrap thesample around the rod. Tape may be used to further secure the samplestrip ends to the glass rod. The rod is then placed in an apparatus fortemperature cycling and examined under optical microscope for extent ofcracking. A scale of 1 to 5 is used to rate the extent of crackingobserved after thermal cycling. The temperature cycling is as follows:

Initial Values of Each Step Graded Target Time Step Tempera- Humidi-Tempera- Humidi- Setting No. ture (° C.) ty (%) ture (° C.) ty (%)(hours) 1 Room Room 20 20 1 2 20 20 2 3 20 20 60 20 1 4 60 20 2 5 60 2060 80 1 6 60 80 2 7 60 80 −10 0 1 8 −10 0 2

In order to further illustrate the present invention, the followingexamples are given. However, it is to be understood that the examplesare for illustrative purposes only and are not to be construed aslimiting the scope of the present invention.

EXAMPLES Examples 1-14

A solution of colloidal elongate silica in isopropyl alcohol (SnowtexIPA-ST-UP from Nissan Chemical Industries, 15% by wt. SiO₂ inisopropanol) is coated onto retroreflective sheeting (Avery DennisonT-7500 Prismatic Grade Reflective Sheeting) or a polyethyleneterephthalate (PET) substrate at various coat weights (wet) and atvarious percent solids of the elongate silica is isopropyl alcohol. Thesolution is coated using a Sheen automatic coater with drawbars of 1 and0.5 gauges. The coated substrates are placed in an air convection ovenand heated at 75° C. for 15 minutes. Table 1 below shows the measuredcontact angle and retroreflectivity of the coated substrates.

Also presented in Table 1 is Comparative Example 14 in which elongatesilica in MEK solvent is used to coat a retroreflective sheet. Theresulting coating has a high contact angle. While not wishing to bebound by theory, it is believed that the high contact angle is theresult of the hydrophobization treatment of the silica particles surfacecarried out to enhance the solution stability of the MEK solventsuspension.

TABLE 1 Initial Contact Reflec- % Coat Angle tivity Exam- Sol- Sub-Thickness (5 (cd/lux/ ple Silica ids strate (wet) mil sec.) m²) 1IPA-ST-UP 15 T7500 1 10.4 1234 2 IPA-ST-UP 10 T7500 1 8.1 1280 3IPA-ST-UP 5 T7500 1 10.4 1229 4 IPA-ST-UP 1 T7500 1 30.5 1277 5IPA-ST-UP 0.1 T7500 1 51.2 1172 6 IPA-ST-UP 15 PET 1 10.4 — 7 IPA-ST-UP15 PET 0.5 7.7 — 8 IPA-ST-UP 10 PET 1 4.4 — 9 IPA-ST-UP 10 PET 0.5 4.7 —10 IPA-ST-UP 5 PET 1 10.4 — 11 IPA-ST-UP 5 PET 0.5 5.4 — 12 IPA-ST-UP 1PET 1 11.0 — 13 IPA-ST-UP 1 PET 0.5 21.1 — Comp. MEK-ST-UP 15 T7500 1.039.8 N/A 14

Example 15-18

Colloidal elongate silica particles in isopropyl alcohol (SnowtexIPA-ST-UP) is mixed with spherical silica particles in isopropyl alcohol(Snowtex IPA-ST-MS) in the weight ratios shown in Table 2. The SnowtexIPA-ST-MS silica sol contains 30% by weight silica having an averageparticle width of 17-23 nanometers. The coatings are preparedsubstantially in accordance with the procedure of Ex. 1-14 above. Thecontact angle (10 sec) measured for each of the coated films ispresented in Table 2 below.

TABLE 2 Ratio of elongate/spherical Coat Weight Contact Angle Example(wt. % solids) (g/m²) 10 sec (deg.) 15 90/10 3.1 4.9 16 100/0  2.3 9.017 70/30 3.5 11 18 50/50 3.9 16

The results indicate that a minor amount of spherical silica added tothe elongate particle silica improves the contact angle of the resultingcoating. For comparative purposes, a coating of 100% spherical silica(Snowtex IPA-ST-MS) is prepared and coated onto a retroreflective sheet.However, the coating does not adhere to the retroreflective sheet sothat the contact angle cannot be measured.

Example 19

A hydrolyzate oligomer of 3-glycidoxypropyl trimethoxysilane (GPTMS) isprepared by mixing 5 g of GPTMS with 1.14 g of aqueous HCL (0.12M),sirring the solution for 1 hour, 24 hours and 1 week to give hydrolysisand condensation product of GPTMS. The coating composition is preparedby mixing the hydrolyzed GPTMS (0.66 g) with 5 g IPA-ST-UP and 2.09 g ofa solution of 3,6-dioxa-1,8-octanedithiol (DOT) in isopropyl alcohol.The coating composition is coated onto a retroreflective sheet (T-7500from Avery Dennison) using a 0.5 mil gauge draw bar and an automaticcoater. The coated film is dried at room temperature, and then cured ina convection oven at 75° C. for 1 hour. Table 3 below shows the contactangle and retroreflectivity of the coated substrates with and withoutXenon Weathering (383 hours).

FIG. 2 is a gel permeation chromatography trace that plots intensity asa function of elution time. The numbers noted on the plots reflect themolecular weight at the particular elution time. The intensity indicatesthe concentration and the time indicates the molecular weight. FIG. 2illustrates the degree of hydrolysis of the GPTMS at 1 hour, 24 hoursand 1 week.

TABLE 3 Time for Xenon Avg. Retro- Water Contact Angle Hydrolysis Hoursreflectivity 0 sec 5 sec 10 sec 1 hour 0 1329 28 26 25 383 1043 12 7 624 hours 0 1371 25 24 22 383 1216 9 6 5 1 week 0 1362 23 21 20 383 83811 8 6

Examples 20-31

In Example 20, a hydrolyzate oligomer of 3-glycidoxypropyltrimethoxysilane (GPTMS) is prepared by mixing 5 g of GPTMS with 1.14 gof aqueous HCL (0.12M), stirring the solution for 1 hour to givehydrolysis and condensation product of GPTMS. The coating composition isprepared by mixing the hydrolyzed GPTMS (0.75 g) with 5 g IPA-ST-UP and0.49 g of a 10% by weight solution of Tinuvin 1130 in isopropyl alcohol.Before coating the retroreflective sheet, 2.09 g of a solution of3,6-dioxa-1,8-octanedithiol (DOT) in isopropyl alcohol is addeddrop-wise to the composition mixture. The mixture is stirred anddegassed. The coating composition is coated onto a retroreflective sheet(T-7500 from Avery Dennison) using a 0.5 mil gauge draw bar and anautomatic coater. The coated film is dried at room temperature, and thencured in a convection oven at 75° C. for 1 hour.

Examples 21-31 are prepared substantially in accordance with theprocedure of Example 20, with the exception that the weight ratio ofhydrolyzed GPTMS is varied as is the presence of UV absorber.Additionally, spherical silica is used in place of the elongate silicain Examples 26 to 31. Table 4 below shows the contact angle andretroreflectivity in the cross direction and in the machine direction ofthe coated substrates after Xenon Weathering (935 hours).

TABLE 4 UV Reflectivity % SiO₂:Binder Absorber Contact Angle (cd/lux/m²)Example Silica Solids (w/w solids) (3% solids) (degree/5 sec) (CD/MD) 20IPA-ST-UP 15 1:1 yes — — 21 IPA-ST-UP 15 3:1 yes 10  832/728 22IPA-ST-UP 15 9:1 yes 7.9  1238/1047 23 IPA-ST-UP 15 1:1 no 15.5 1368/1112 24 IPA-ST-UP 15 3:1 no 9.7 1218/989 25 IPA-ST-MS 15 9:1 no8.5 1168/917 26 IPA-ST-MS 15 1:1 yes 17  1500/1237 27 IPA-ST-MS 15 3:1yes 12 1343/960 28 IPA-ST-MS 15 9:1 yes 6.4 1300/986 29 IPA-ST-MS 15 1:1no 16.6 1042/927 30 IPA-ST-MS 15 3:1 no 9.2  1362/1167 31 IPA-ST-MS 159:1 no 9.1  1521/1216

Examples 32-37

Examples 32-37 are prepared substantially in accordance with theprocedure of Example 20, with the exception that the weight ratio ofhydrolyzed GPTMS is varied as is the presence of UV absorber. Examples36 and 37 use elongate silica in MEK in place of the elongate silica inisopropyl alcohol. Table 5 below shows the contact angle andretroreflectivity of the coated substrates.

Examples 38-39

For Example 38, a hydroxyethyl methacrylate (HEMA)/methyl methacrylate(MMA) copolymer solution is prepared by degassing, followed by heatingat 60° C. for 24 hours a mixture of 2.3 g HEMA, 17.70 g MMA (10 mol MMAto 1 mol HEMA) and 0.05 g Vazo 64 in 80.0 g dry MEK. The coatingcomposition is prepared by mixing 2.00 g of the polymer solution and0.018 g aluminum acetylacetonate (AAA, 3% by weight with respect tosolids) and 2.0 g Snowtex MEK-ST-UP. The coating composition is coatedon retroreflective sheeting (Avery Dennison T-7500) using a 0.5 milgauge draw bar and an automatic coater. The coated film is dried at roomtemperature and then cured in a convection oven at 75° C. for 1 hour.

Example 39 is prepared substantially in accordance with the procedure ofExample 38, with the exception that weight ratio of silica to organicbinder is varied.

Table 5 below shows the contact angle and retroreflection of theresulting coated substrates. Also shown is the contact angle andretroreflection of the coated substrates of Examples 21-35 after theyhave been subjected to corona treatment.

TABLE 5 Contact Angle Reflectivity UV (degree/5 sec) (cd/lux/m²) %SiO₂:Binder Absorber Contact Angle Reflectivity with corona with coronaExample Silica Solids (w/w) (3% solids) (degree/5 sec) (cd/lux/m²)treatment treatment 32 IPA-ST-UP 10 1:1 yes 14.7 1285/1296 13.91260/1247 33 IPA-ST-UP 10 3:1 yes 27.7 1276/1319 9.1 1211/1293 34IPA-ST-UP 10 9:1 yes 18.8 1397/1364 6.9 1255/1293 35 IPA-ST-UP 10 1:0yes 14.9 1314/1364 6.2 1273/1307 36 MEK-ST-UP 15 1:1 no 56.6 1129/1227 —— 37 MEK-ST-UP 15 9:1 no 65.9 1122/1200 — — 38 MEK-ST-UP 15 1:1 no 62.71194/1090 — — 39 MEK-ST-UP 15 9:1 no 25.4 1168/1124 — —

Examples 40-43a

Examples 40-43a are directed to dual layer dew resistant coatings.Specifically, a first primer layer is formed on the retroreflectivesheet, followed by a second top layer formed over the primer layer.

Preparation of Primer A:

Primer A is prepared by mixing together in a 1:1 ratio by weight3-glycidoxypropyl trimethoxysilane (GPTMS) and Snowtex IPA-ST-MSspherical particles.

Preparation of Primer B:

Primer B is prepared by mixing together in a 4:1 ratio by weight amethyl methacrylate/methoxypropyltrimethoxysilane copolymer (7.65:1MMA:MOPTS) with hydrolyzed tetraethoxysilane.

Preparation of Primer C:

Primer C is prepared by mixing together in a 1:1 ratio by weight3-glycidoxypropyl trimethoxysilane (GPTMS) and Snowtex IPA-ST-UPelongate particles.

Top Layer I:

The composition of Top Layer I is Snowtex IPA-ST-UP elongate particles.

Top Layer II:

The composition of Top Layer II is a photocatalyst solution of TiO₂ witha solids content of 9.4% (Bistrator NRC-300C from Nippon Soda Co.,Ltd.).

Example 40 Control

Top Layer I is coated onto a retroreflective sheet (Avery T-7500) at acoat thickness of 1 mil (wet) and heated in a convection oven at 70° C.for 30 minutes. The contact angle and retroreflectivity of the coatedsheet is shown below in Table 6 below.

Example 41

Primer A is coated onto a retroreflective sheet (Avery T-7500) at a coatthickness of 1 mil (wet) and heated in a convention oven at 55° C. for15 minutes. Top Layer I is then applied over Primer Layer A at a coatthickness of 1 mil (wet) and heated for at 70° C. for 1 hour. Thecontact angle and retroreflectivity of the coated sheet is shown belowin Table 6 below.

Example 42

Primer B is coated onto a retroreflective sheet (Avery T-7500) at a coatthickness of 1 mil (wet) and heated in a convention oven at 70° C. for 1hour. Top Layer I is then applied over Primer Layer B at a coatthickness of 1 mil (wet) and heated for at 70° C. for 1 hour. Thecontact angle and retroreflectivity of the coated sheet is shown belowin Table 6 below.

Example 43

Primer C is coated onto a retroreflective sheet (Avery T-7500) at a coatthickness of 1 mil (wet) and heated in a convention oven at 55° C. for15 minutes. Top Layer II is then applied over Primer Layer C at a coatthickness of 1 mil (wet) and heated for at 70° C. for 1 hour. Thecontact angle and retroreflectivity of the coated sheet is shown belowin Table 6 below.

Example 43a

Example 43a is substantially the same as Example 43, with the exceptionthat prior to measuring the contact angle and retroreflectivity, thecoated substrate is placed in a Xenon Weatherometer overnight for UVactivation of the Top Layer II. Subsequent to UV activation of thephotocatalytic layer, the contact angle of the coated substratedecreases relative to that Example 43, the un-activated coatedsubstrate.

TABLE 6 Exam- Prim- Top Contact Angle degree (std. dev.) Retro- ple erLayer 0 sec 5 sec 10 sec reflectivity 40 — I 6.5 (0.6) 5.1 (0.5) 4.1(0.2) 1347 41 A I 21.5 (1.6) 19.2 (3.1) 18.3 (1.0) 1309 42 B I 12.1(0.2) 5.2 (0.2) 4.0 (0.3) 1288 43 C II 46.4 (4.3) 45.5 (2.4) 45.4 (2.3)1069 43a C II 5.6 (0.6) 4.5 (0.8) 3.3 (0.5) 1248

Examples 44-49

A coating of 100% elongate silica (Snowtex IPA-ST-UP) is applied toretroreflective sheeting at various coat weights. The coated sheeting issubjected to the Mandrel Test described above. Table 7 shows the resultsof the testing.

TABLE 7 Example Coat Weight (gsm) Mandrel Rating* 44 0.57 0 45 0.81 1 461.12 3 47 1.13 2 48 1.27 3 49 2.85 5 *A rating of 0 indicates no cracks,1 indicates some tiny cracks, 2 indicates thin and light cracks, 3indicates moderate cracks, 4 indicates dense and thin cracks, 5indicates dense and thick cracks.

Example 50

A hydrolyzate oligomer of 2-(3,4-epoxycyclohexyl)ethyl trimethoxysilane(CHTMS) is prepared by mixing 3 gms of CHTMS with 3.67 gms ofisopropanol and 0.66 gms of aqueous HCl (0.12M), stirring the solutionfor 1 hour to obtain the hydrolysis and condensation product of CHTMS.The coating composition is prepared by mixing the hydrolyzed CHTMS (0.24gms) with 6 gms of elongate silica particles in isopropyl alcohol(Snowtex IPA-ST-UP) and 0.03 gms[4-[(2-hydroxytetradecyl)oxy]-phenyl]phenyliodonium hexafluoroantimonate(a cationic UV initiator). The total solids content is decreased to afinal 10% by weight by the addition of 3.76 gms of coating solventisopropanol.

The coating composition is coated onto a retroreflective sheet (T-7500from Avery Dennison Corporation) using a 0.5 mil gauge draw bar and anautomatic coater. The coated film is dried at 70° C. for 2 minutes, andthen UV cured using a Fusion UV System with an H-bulb at 35 fpm for 1pass. The film is then corona treated prior to testing.

Examples 51-54

Dew resistant coatings are prepared substantially in accordance with theprocedure of Example 50, with the exception that the amount ofhydrolyzed CHTMS used is varied as shown in Table 8. The weight percentCHTMS shown is based on the total solids of the coating composition. Therefractive index, obtained by the ellipsometry method for the coatingsand the retroreflectivity of retroreflective sheets coated with thedew-resistant coatings as compared to the uncoated retroreflectivesheets (two samples of each coating) are shown in Table 8. Theretroreflectivity was measured in the machine direction (MD) and thecross direction (CD) for each sample.

TABLE 8 % wt. Refractive Uncoated After Coating Example CHTPMS Index MDCD MD CD 50 20 1.29 1177 1152 1218 1210 1137 1153 1208 1216 51 5 1.351120 1115 1244 1255 1101 1140 1199 1237 52 7 1.38 1054 1119 1302 12991103 1121 1232 1257 53 50 1.49 1134 1159 1207 1254 1139 1156 1299 128554 10 — — —

Example 55

A dew resistant coating is prepare substantially in accordance with theprocedure of Example 50, with the exception that the hydrolysis andcondensation reactions of CHTMS are carried out by reacting the monomerCHTMS in an aqueous solution of elongate silica particles followed byvacuum distillation to remove the water and subsequent dilution withisopropanol.

Example 56

A hydrolyzate oligomer of 2-(3,4-epoxycyclohexyl)ethyl trimethoxysilane(CHTMS) is prepared by mixing 3 gms of CHTMS with 3.67 gms ofisopropanol and 0.66 gms of aqueous HCl (0.12M), stirring the solutionfor 1 hour to obtain the hydrolysis and condensation product of CHTMS.The coating composition is prepared by mixing the hydrolyzed CHTMS (0.24gms) with 6 gms of elongate silica particles in isopropyl alcohol(Snowtex IPA-ST-UP) and 0.03 gms 3,6-dioxa-1,8-octanedithiol (DOT). Thetotal solids content is decreased to a final 10% by weight by theaddition of 3.76 gms of coating solvent isopropanol. The coatingcomposition is coated onto a retroreflective sheet (T-7500 from AveryDennison Corporation) using a 0.5 mil gauge draw bar and an automaticcoater. The coated film is dried at 70° C. for 2 minutes, and then curedin a convection oven at 75° C. for 1 hour.

Table 9 below shows the percent blackout for examples of the dewresistant coating that were coated onto retroreflective sheeting (AveryDennison T-7500).

TABLE 9 Percent Blackout Days Uncoated Example 1 Example 51 Example 52 0100% 40% 86% 86% 4 100% 67% — — 6 100% 38% — — 13 100% 45% — — 22 100%50% — — 25 100% — 46% 54% 27 100% — 33% 33% 29 100% — 52% 52% 34 100% — 9%  9% 36 100% — 15% 15% 48 100% 31% — — 50 100% 27% — — 52 100% 67% —— 57 100% 25% — — 59 100% 27% — —

The dew resistant coatings of Examples 1, 50, 52 and 54 were evaluatedfor durability. Specifically, the retroreflectivity and contact anglefor these coatings on a retroreflective sheet (T-7500 from AveryDennison Corporation) prior to exposure and after 4502 hours of Xenonweathering are Shown in Table 10 below.

TABLE 10 Retroreflectivity Contact Angle Before 4502 hrs Before 4502 hrsExample exposure Xe exposure Xe 1 1310 1237 4.7 6.0 50 1010 1124 2.8 8.852 1192 1119 3.5 11.6 54 1112 1160 4.5 10.1

The durability of dew resistant coatings of Examples 50 and 51 wereevaluated on various retroreflective substrates as shown in Table 11below. Specifically, the contact angle was measured for samples coatedon Retroreflective sheeting (T-7500 from Avery Dennison Corporation)before and After 2120 hours of Xenon weathering.

TABLE 11 Contact Angle Contact Angle Before Exposure After 2120 hoursExample Substrate [deg. (std. dev.)] [deg. (std. dev.)] 50 white 25.3(1.7) 7.2 (1.0) 50 blue 26.7 (1.2) 7.7 (2.0) 50 green 26.4 (0.5) 5.9(1.0) 51 white 25.0 (0.8) 9.5 (3.0) 51 blue 25.7 (0.9) 10.7 (1.0) 51green 25.8 (0.2) 11.9 (1.0)

Although the invention has been shown and described with respect to acertain embodiment or embodiments, it is obvious that equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification. In particularregard to the various functions performed by the above describedelements (components, assemblies, compositions, etc.), the terms used todescribe such elements are intended to correspond, unless otherwiseindicated, to any element which performs the specified function of thedescribed element (i.e., that is functionally equivalent), even thoughnot structurally equivalent to the disclosed structure which performsthe function in the herein illustrated exemplary embodiment orembodiments of the invention. In addition, while a particular feature ofthe invention may have been described above with respect to only one ormore of several illustrated embodiments, such feature may be combinedwith one or more other features of the other embodiments, as may bedesired and advantageous for any given or particular application.

1. A retroreflective article comprising a substrate having aretroreflective surface thereon, and a coating overlying theretroreflective surface, the coating comprising (a) at least about 75%by weight of elongate silica particles having an aspect ratio greaterthan 1, and (b) an organic binder comprising at least one hydrolysisproduct or at least one partial condensate of an epoxy functionalsilane, wherein the coating comprises about 15 to 25% by weight of anorganic binder.
 2. The article of claim 1 wherein the silane is selectedfrom glycidoxy methyltrimethoxysilane,3-glycidoxypropyltrihydroxysilane,3-glycidoxypropyl-dimethylhydroxysilane,3-glycidoxypropyltrimeth-oxysilane, 3-glycidoxypropyltriethoxysilane,3-glycidoxypropyl-dimethoxymethylsilane,3-glycidoxypropyldimethylmethoxysilane,3-glycidoxypropyltributoxysilane,1,3-bis(glycidoxypropyl)tetramethyldisiloxane,1,3-bis(glycidoxypropyl)tetramethoxydisiloxane,1,3-bis(glycidoxypropyl)-1,3-dimethyl-1,3-dimethoxydisiloxane,2,3-epoxypropyl-trimethoxysilane, 3,4-epoxybutyltrimethoxysilane,6,7-epoxyheptyl-trimethoxysilane, 9,10-epoxydecyltrimethoxysilane,1,3-bis(2,3-epoxypropyl) tetramethoxydisiloxane,1,3-bis(6,7-epoxyheptyl)tetramethoxydisiloxane,2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and combinations of two ormore thereof.
 3. The article of claim 1 wherein the coating furthercomprises an organosilane compound not having epoxy functionality.
 4. Aretroreflective article comprising a substrate having a retroreflectivesurface thereon, and a coating overlying the retroreflective surface,the coating comprising (a) at least about 75% by weight of elongatesilica particles having an aspect ratio greater than 1, and (b) anorganic binder comprising at least one hydrolysis product or at leastone partial condensate of silane compound, wherein the silane compoundcomprises cyclohexyltrimethoxysilane.
 5. A retroreflective articlecomprising a substrate having a retroreflective surface thereon, and acoating overlying the retroreflective surface, the coating comprising(a) at least about 75% by weight of elongate silica particles having anaspect ratio greater than 1, and (b) an organic binder comprising atleast one hydrolysis product or at least one partial condensate of anepoxy functional silane, wherein the coating further comprises sphericalsilica particles having an average diameter of less than 50 nanometers.6. A retroreflective article comprising a substrate having aretroreflective surface thereon, and a coating overlying theretroreflective surface, the coating comprising (a) at least about 75%by weight of elongate silica particles having an aspect ratio greaterthan 1, and (b) an organic binder comprising at least one hydrolysisproduct or at least one partial condensate of an epoxy functionalsilane, wherein the coating further comprises titanium dioxidenanoparticles.
 7. A retroreflective article comprising a substratehaving a retroreflective surface thereon, and a coating overlying theretroreflective surface, the coating comprising (a) at least about 75%by weight of elongate silica particles having an aspect ratio greaterthan 1, and (b) an organic binder comprising at least one hydrolysisproduct or at least one partial condensate of an epoxy functionalsilane, wherein the silica particles comprise chain silica particles.